Restored ability to drop on element attribute

Drop feedback was breaking it.
Left and top updated during drag

If element widget's backside is displaying left or top attribute
Added a confirmation message when quitting

Fixes #59.
Add backup to localStorage

Also fixed a bug creating the top-level backside during initialisation.
Removal of backside widgets recorded

So backup to local storage does not include backside widgets that have
been removed.

Also added more actions that cause backup.
Fixed a bug in maintaining backside_widgets

Was getting duplicates.
Backup delayed slightly

In order for the geometry to settle down.

Also fixed a just introduced bug in adding multiple backside widgets.
Merge pull request #62 from ToonTalk/master

Updating gh-pages to master